1. Field of the Invention
The present invention relates in general to AC power control systems, and more particularly to power control methods and apparatus for controlling the AC power delivered to a laser printer fuser.
2. Description of the Related Art
Different types of reproduction equipment employ fusers to permanently fuse toner particles onto a print medium, such as paper, to generate characters and images on the print medium. Examples of such reproduction equipment include copiers, printers, scanners, facsimile machines, and other well known equipment. The equipment receives data representative of the characters or image to be reproduced onto the print medium. Programmed circuits receive the data and apply an electrostatic charge to a print drum, whereupon the toner particles are attracted to the drum at the locations forming the characters or image. As the print medium passes over the drum, the toner particles are transferred to the print medium. The print medium then passes through a fuser that rapidly heats the toner and the paper, and with pressure the toner is melted and pressed into or onto the print medium.
The fuser requires substantial electrical power to bring the apparatus up to operating temperature and to rapidly heat the print medium during the reproduction process. Indeed, the power used to heat typical fusers can be 500-1,000 watts. During the reproduction process, the thermal energy needs of the fuser require power to be applied thereto when needed to maintain the fuser apparatus at a relatively constant temperature. To that end, most reproduction equipment employing fusers use a power control circuit which delivers electrical energy to the fuser, a temperature sensor to monitor the fuser temperature, and a programmed controller to control the overall reproduction and fusing process.
Most reproduction equipment use the AC line power to heat the fuser. The on and off cycling of AC power to the fuser can cause voltage fluctuations on the AC power line. In view of the wattage requirements of fusers, the on and off cycling of the AC power to the fuser can cause undesired operation of other equipment which also uses AC power from the same power line. For example, incandescent lights connected to the same AC power line may flicker, which is annoying. In some instances, if the fluctuation in the AC line voltage is sufficient, fluorescent lights can be extinguished. Also, some types of AC control circuits for fusers cause the generation of electrical harmonics which, when reflected back onto the AC power line, can also cause undesired operation of other equipment using the AC power. Often various governmental regulations require that the flicker and harmonics generated by reproduction equipment fusers be maintained at minimum specified levels.
In U.S. Pat. No. 6,847,016 entitled “System And Method For Controlling Power In An Imaging Device,” the system converts the AC power into a DC power and drives multiple heaters for heating the fuser. The control system heats multiple heating elements of a fuser in a temporally-shifted manner to create an effective drive frequency that exceeds an actual drive frequency at which the heating elements are driven.
In U.S. Pat. No. 6,111,230, entitled “Method And Apparatus For Supplying AC power While Meeting The European Flicker And Harmonic Requirements,” AC power is applied to the fuser by using phase angle techniques to apply only a portion of the AC power in each AC cycle until power is ramped up, and then using the full cycle AC power during the remainder of the heating cycle. The duration of the application of the full cycle AC power determines the steady state heat delivered to the fuser. This technique is a hybrid between phase angle control of the AC power during initial turn on of the fuser, and full cycle control during the remainder of the fuser power cycle.
In the reproduction equipment industry, there other popular methods to switch the input AC line voltage to a fuser. One technique is an integer half cycle control and the other technique is the phase angle control method, noted above. The integer half cycle control is illustrated in FIG. 1. According to this technique, the AC power control circuit outputs full half cycles of AC power to be coupled to the fuser heater. An AC switch in the control circuit turns on and off at the zero crossing and allows half cycles of the AC power to be coupled to the fuse heater. At the zero crossing points in time, the surge current coupled to the fuser is very small, thus resulting in a low harmonic content generated and reflected back into the AC power line. The same number of positive half cycles and negative half cycles are used, resulting in a zero DC offset in the AC current. While not shown, the AC switch can also be turned on at the start of a negative half cycle, as well as the start of the succeeding positive half cycle. This type of AC power control operates at a relatively low frequency, as some half cycles are used and other half cycles are not used. With a fuser powered using the integer half cycle technique, and operating at 25% power, the line voltage may fluctuate at an effective 15 Hz rate, as one full cycle is used out of every four full cycles of a 60 Hz line frequency. The 15 Hz power fluctuation may cause objectionable flicker in an incandescent lamp connected to the same AC power line.
According to another AC power control technique employed with reproduction equipment fusers, a higher frequency is utilized, where the AC switch is triggered during a partial half cycle. Typically the AC switch which controls the AC power delivered to the fuser is enabled at the same point during each half cycle, referred to as the phase angle. The phase angle technique is illustrated in FIG. 2. The rising edge of the enable signal causes the AC switch to close and to immediately couple the AC power to the fuser heater. The AC switch remains enabled during the remainder of the AC cycle until a subsequent zero crossing is sensed, whereupon the AC switch automatically opens. The partial AC cycles are output to the fuser heater, resulting in no DC offset of the AC line current. The power ratio is more difficult to calculate, as the power varies as the square of the switched sinusoidal voltage waveform. FIG. 3 illustrates the relationship between the time enable signals (delayed from a zero crossing), and the output power for a cycle with a period T in the phase angle technique. If the delay is zero, the enable signal is active at the zero crossing time and 100% power is delivered. At a delay of 4/5 of the half cycle, i.e. 8 ms at 50 Hz, the power ratio is about 5%, as opposed to the 20% level that would be expected if the power were proportional to the enable time. The resulting higher frequency power fluctuations rarely cause a visual flicker with incandescent lights using the same power line voltage. However, because the switch is actuated during non-zero crossings of each half cycle (positive and negative) of the AC voltage, there is a harmonic rich turn-on transition as the line voltage is connected to a low impedance load of the fuser heater. The harmonic content is reflected back into the input AC line and can cause the printer to fail governmental standards and regulations, and can cause unreliable operation of other equipment connected to the same AC power line.
Both the half cycle control and the phase angle control techniques are required to be applied properly to generate the same number of positive half cycles and negative half cycles of the AC power. When properly applied in practice, there should be a nominal DC offset of zero AC line current, which is also controlled by regulations.